A wireless local area network (WLAN) uses radio frequency (RF) signals to transmit and receive data between electronic devices. WLANs provide all of the features and benefits of traditional hard-wired LANs without requiring cable connections between the devices. Referring now to FIG. 1, an infrastructure-based WLAN 10 includes a wireless access point 11 that provides a transparent connection between stations 12-1, 12-2, . . . , and 12-n and a network 14. The network 14 typically includes a distributed communication system 16 such as an Ethernet and one or more servers 18.
The access point 11 is the wireless equivalent of a hub. The access point 11 communicates with the wireless stations 12 using antennas 22. The access point 11 maintains the connections to the stations 12 that are located in a coverage area 24. The access point 11 also typically handles security by granting or denying access to the network 14. Similarly, a wireless access point 32 provides a transparent connection between stations 34-1, 34-2, . . . , and 34-n and the network 14. The access point 32 also communicates with the wireless stations 34 using antennas 42. The wireless access point 32 maintains the connections to stations 34 that are located in a coverage area 42.
Referring now to FIG. 2, an independent WLAN 38 supports direct wireless communications between stations 40-1, 40-2, . . . , and 40-n in a coverage area 42. Referring now to FIG. 3, stations 50 include a controller 52, and a transceiver 54 that is connected to one or more antennas 56. The stations 50 include additional circuits 58 for processing transmit/receive signals and for performing various other common functions of stations 50. Referring now to FIG. 4, access points 60 also include a transceiver 64 that is connected to one or more antennas 66. Likewise, the access point 60 includes additional circuits 68 for processing transmit/receive signals and for performing various other common functions of access points 60.
Referring now to FIG. 5, a WLAN architecture 70 for stations 50 and the access point 60 is shown. The architecture 70 includes a media access control (MAC) sublayer 72 that communicates with a MAC layer manager 74. A physical layer convergence protocol (PLCP) sublayer 76 and a physical medium dependent (PMD) sublayer 78 communicate with a physical layer manager 80. IEEE sections 802.11, 802.11(a), and 802.11(b), which are hereby incorporated by reference, set forth other specifications and operating details of the WLAN architecture 70.
When multiple stations are located in a coverage area, they compete for access to a medium. In other words, only a single station can transmit data or acknowledge receipt of data at a time. Standards such as IEEE sections 802.11, 802.11(a), and 802.11b set forth a specific protocol for WLAN communications to accommodate contention between the devices for the medium.
Referring now to FIG. 6, a source station 90 transmits data 92. Following a short interframe space (SIFS), a destination station 96 generates an acknowledgment (ACK) 98. IEEE section 802.11 and other related sections set forth a maximum time interval for the destination station 96 to respond with the ACK 98. Other stations 100 must defer access during the transmission of the data 92 and for a period that is longer than the SIFS to avoid contention.
Referring now to FIG. 7, the SIFS is shown in further detail. The SIFS is defined as a nominal time that the MAC and PHY require to receive the last symbol of a frame, to process the frame, and to respond with a first symbol of a response frame. In other words, aSIFSTime=aRXRFDelay+aRXPLCPDelay+aMACProcessingDelay+aRxTxTurnaroundTime. For both 802.11 and 802.11(b), the nominal aSIFSTime is defined as 10 microseconds (μs).
In FIG. 7, D1 is equal to aRXRFDelay+aRXPLCPDelay, which corresponds to a receiver delay and a receiver processing delay, respectively. Ml is equal to aMACProcessingDelay, which is the processing delay of the MAC layer. RxTx is equal to the RxTxTurnaroundTime, which is the delay associated with a transition between receiver and transmitter modes.
In implementations that comply with IEEE section 802.11 and related sections, aSIFSTime must not vary from the defined nominal SIFS time value by more than 10% of the slot time. For both sections 802.11 and 802.11(b), aSIFSTime must be between 8 and 12 μs as measured on the medium to be compatible. These limitations on aSIFSTime prevent the use of advanced signal processing techniques. For example, Turbo coding, Reed-Solomon coding, convolutional code concatenated with Reed-Solomon coding, and other advanced error coding techniques are not possible when backward compatibility is required. These advanced signal processing techniques allow higher data rates and/or provide other advantages. However, the advanced signal processing techniques also require receiver processing time that is typically longer than the nominal aSIFSTime that is defined by IEEE 802.11 and related sections.